The negative impacts of cigarettes, on both smokers and those around them, are widely known. While some effects may only be cosmetic (wrinkling, yellowing of the skin), others, such as cancer, can be fatal. According to a team of Mexican researchers, however, cigarette smoking can be beneficial to at least one group: urban birds. The benefit, unsurprisingly, does not come from actually smoking the cigarettes, but instead from incorporating discarded butts into nests. There, chemical residues in the cigarette fibers appear to act as repellents that keep parasites away from vulnerable nestlings
(Male house sparrow, Passer domesticus. Image courtesy of Redbridge Birdwatching.)
These unusual and surprising findings, published in the journal Biology Letters, were the result of an undergraduate research project conducted on the campus of the National University of Mexico in Mexico City. The project was undertaken after researchers noted the presence of cigarette butts in nests of urban-dwelling house finches (Carpodacus mexicanus) and house sparrows (Passer domesticus). Similar patterns have also been reported in other species and locations, prompting the scientists to wonder whether the inclusion of the butts was deliberate. This seemed particularly likely given the fact that nicotine, one of the dominant chemicals in cigarettes, is known to repel ectoparasites—parasites that live on the surface of the body. In fact, both poultry and rabbit farmers use nicotine as an organic method of parasite control.
To investigate whether the urban birds had also begun to employ nicotine as a pesticide, the researchers performed both experimental and observational work. For the experimental portion of the study, nests of both species were located and fitted with thermal traps designed to attract ectoparasites by mimicking the heat signatures of potential hosts. Cellulose cigarette fibers—from either smoked or unsmoked cigarettes—were attached to the heating component of each nest, and a strip of adhesive was placed next to the heaters in order to capture all approaching parasites. The researchers hypothesized that they would find fewer parasites near the heaters fitted with smoked cigarette fibers, since these materials contained a higher proportion of pesticidal chemicals. Indeed, they found that parasites were not only more scarce at these nicotine-treated sites, but, in some cases, were nearly 6 times less likely to appear.
(House finch, Carpodacus mexicanus. Image courtesy of Wikimedia.)
Once the experiments were complete, the research team waited until the birds had finished breeding and then returned to harvest the empty nests. These were weighed and then dissected; each nest’s cellulose content was measured, and all ectoparasites were counted and identified. Assuming that cigarette butts act as a repellent, the scientists expected to find fewer parasites in nests with more butts. The majority of nests of both house sparrows and house finches contained bits of discarded cigarettes. Interestingly, while butts were more likely to be found in nests of the former (89% vs. 86%), they were found in higher quantities in nests of the latter (an average of 10 vs. 8 per nest). Both species suffered from similar rates of parasite infestation, and, as predicted, parasite abundance was negatively related to the presence of cigarette fibers.
The results indicate that the nicotine-laced cellulose deters parasites—and, further, suggest that the birds may be selecting these building materials on purpose. You might think that would require a bit more thoughtfulness than is possessed by the average songbird, but this is not the first time they have shown such an ability. Several other species, including both European starlings (Sturnus vulgaris) and tree swallows (Tachycineta bicolor), are known to deliberately incorporate green plant materials into their nests; like cigarettes, these botanical building blocks contain compounds that repel parasites.
Thus, the use of the cigarette butts appears to be “an urban manifestation of a pre-existing behavior”—or, to quote the title of the paper in which these results were presented, the use of “new ingredients for an old recipe.” The sparrows and finches appear to be using the recipe to “self-medicate,” though for this to be proven definitively, further work will be required. Specifically, while it is clear that the cigarette butts reduce the likelihood of parasite infestation, it is still necessary to show that the birds are purposely selecting these items for inclusion in their nests, and that use of the butts improves the birds’ reproductive success.
The research team has suggested several additional studies that could help them identify whether the cellulose might serve another purpose (such as providing insulation) and also whether the toxic residues in the fibers might have negative effects on nestlings. These data will reveal whether—in the case of birds, at least—cigarettes can sometimes do more good than harm.
Suarez-Rodriguez, M., Lopez-Rull, I., and Garcia, M.C. 2013. Incorporation of cigarette butts into nests reduces nest ectoparasite load in urban birds: new ingredients for an old recipe? Biology Letters 9(1): online advance publication.
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